In recent years, to meet demands for increased shift comfort, improved driveability, and reduced fuel consumption and exhaust emissions, there have been proposed and developed toroidal continuously variable transmissions often abbreviated to “toroidal CVTs”, in which a transmission ratio is steplessly variable within limits. One such toroidal CVT has been disclosed in Japanese Patent Provisional Publication No. 10-331938 (hereinafter is referred to as “JP 10-331938”), corresponding to U.S. Pat. No. 6,030,309. On such a toroidal CVT as disclosed in JP 10-331938, engine power (output torque) is transmitted from an input disk to an output disk via a traction oil film formed between a power roller and each of the input and output disks, using a shearing force in the traction oil film at high contact pressure. The input and output disks coaxially oppose each other. The toroidal CVT has a trunnion serving as a power roller support that rotatably supports the power roller, which is interposed between the input and output disks and is in contact with a torus surface of each of the input and output disks under preload. During transmission-ratio changing of the toroidal CVT, in order to obtain a desired transmission ratio determined based on the magnitude of a gyration angle of the power roller, the power roller is shifted from a neutral position, at which a rotation axis of the power roller intersects the center of rotation (common rotation axis) of the input and output disks, by slightly shifting or displacing the trunnion in a direction of a trunnion axis perpendicular to the rotation axis of the power roller via a servo piston of a hydraulic servo mechanism that operates in response to a hydraulic pressure generated by a prime-mover driven oil pump that is constantly driven by a prime mover (an engine) during operation of the prime mover. In more detail, during a forward running mode the hydraulic pressure is directed to the hydraulic servo mechanism via a forward ratio control valve, whereas during a reverse running mode the hydraulic pressure is directed to the hydraulic servo mechanism via a reverse ratio control valve. By virtue of a side slip force occurring in a very limited contact zone between the power roller and the input and output disks due to the slight offset or slight vertical displacement of the power roller, the power roller is self-tilted or self-inclined. Owing to the self-inclining motion of the power roller, a first diameter of a circular-arc shaped locus drawn by movement of the very limited contact point between the power roller and the output disk on the torus surface of the output disk and a second diameter of a circular-arc shaped locus drawn by movement of the very limited contact point between the power roller and the input disk on the torus surface of the input disk, that is, a ratio of the first to second diameter can be continuously varied, thus continuously varying a transmission ratio. Generally, in the toroidal CVT, a degree of progress for transmission-ratio changing is fed back to the hydraulic servo mechanism, so that the trunnion gradually returns to its initial position as the transmission-ratio changing progresses. When the gyration angle based on a desired transmission ratio corresponding to a transmission-ratio command signal value has been reached, the vertical displacement of the trunnion is returned to zero, so as to terminate the inclining motion of the power roller, and to attain the return of the power roller to neutral, and thus to maintain the desired transmission ratio corresponding to the ratio command signal value.
In the previously-noted prime-mover driven oil pump, pressurized working fluid (pressurized traction oil) is continuously discharged from the pump during operation of the prime mover. Therefore, during operation of the prime mover, the hydraulic servo mechanism is controllable by way of hydraulic pressure produced by the prime-mover driven oil pump. However, suppose that torque backwardly flows from road wheels to the output disk owing to hauling or coasting in a stopped state of the prime mover in which there is no hydraulic pressure produced by the prime-mover driven oil pump and needed for ratio change control, and thus the hydraulic servo mechanism is in an uncontrolled state. In such a case, there is an increased tendency for the toroidal CVT to undesirably shift up, for the reasons discussed above.
When the output disk is driven by road wheels due to back-flow of torque from the road wheels to the output disk, as a push-back force or a reaction force from a contact portion between the power roller and the input shaft, the power roller, interposed between the input and output disks under preload, receives a component force acting in the trunnion-axis direction. This causes a slight offset of the power roller from its neutral position in the trunnion-axis direction that upshifts the toroidal CVT to a higher transmission ratio. As a result of this, owing to the self-inclining motion of the power roller, the upshift of the toroidal CVT to a higher transmission ratio undesirably occurs. If the prime mover is restarted and the vehicle is accelerated from standstill on the assumption that the toroidal CVT has been undesirably upshifted to a high transmission ratio owing to hauling or coasting in the stopped state of the prime mover, there are the following drawbacks.
During early stages of vehicle starting, a desired transmission ratio corresponding to a transmission-ratio command signal is generally set at a predetermined lowest ratio. As is generally known, ratio changing operation can be made only in presence of torque transmission after the vehicle is started. As a matter of course, just before starting the vehicle, there is no torque transmission, and thus the vehicle is restarted at the transmission ratio remaining high on the assumption discussed above. This is often called as a “high-ratio starting”. During the so-called high-ratio starting, there is a lack in torque, thus deteriorating the starting performance of the vehicle. To avoid the high-ratio starting, Japanese Patent Provisional Publication No. 2002-327836 (hereinafter is referred to as “JP2002-327836”), corresponding to United States Patent Application Publication No. US 2002/0169051 A1, published Nov. 14, 2002, teaches the use of a secondary oil pump as well as a primary oil pump (a prime-mover driven oil pump).